The Effect of Heat Treatments on the Mechanical Properties of Expired Endodontic Gutta-percha – An in vitro Study

Abstract

Introduction:

Expired gutta-percha (GP) can lose its mechanical properties. This study aimed to compare the tensile strength values between expired and nonexpired GP-cones and to assess the outcome of different heat temperatures on this strength.

Materials and Methods:

Two-hundred-forty ProTaper points were obtained from nonexpired and 2 years expired pack with two different subgroups (sealed or exposed to air and light). All groups were subjected to the following treatments: control, 90°C water bath and cool either at room temperature or 0°C and 70°C water bath and cool either at room temperature or at 0°C. The strength under the tensile load was calculated using the universal testing machine.

Results:

The collected data were evaluated by one-way analysis of variance. Tensile strength for all control groups did not show any difference. Significant increase in the tensile strength in groups heated for 1 min in a water bath at 70°C, the tensile strengths were 8.19 ± 2.72 Mpa and 5.94 ± 1.14 Mpa cooled at 25°C and 0°C, respectively, P ≤ 0.001. Two years expiration did not lower the tensile strength of the GP and exposure to air and light did not show any change. Appropriate storage of GP is a key factor to extend its lifespan.

Conclusion:

A clinician may consider using expired GP as long as there are no noticeable changes in their structure and properties. A simple chairside technique using hot water may be helpful in restoring the mechanical properties of expired GP.

INTRODUCTION

Gutta-percha (GP) is a common endodontic filling material.[1] It mainly composes of 1, 4 trans-isomer polyisoprene.[2] GP is a viscoelastic and thermoplastic filling material.[2] It is also a temperature-sensitive material that presents in a stiff and solid form at room temperature and turns soft around 60°C; on further heat, it melts with partial degradation at 95°C–100°C.[3] GP polymer presents in different forms: the alpha and the beta crystalline forms. The “alpha” is a natural form which is present in trees. This form expresses high flowability and low viscosity when heated.[4] It also shows an excellent adhesion property compared to the beta form.[5] In particular, commercially available GP cones are present in the “beta” form that has a low flowability and higher viscosity.[6] If alpha GP reaches 65°C, it transforms into an amorphous phase and melts. If the amorphous form cools rapidly, beta form recrystallizes, while if it cools slowly (0.5°C/h), alpha form recrystallizes, whereas the beta form undergoes two endothermic peaks: from beta phase to alpha phase if the temperature rises between 42 and 49°C and from alpha phase to amorphous phase if temperature between 53 and 59°C.],[4,5,6,7]

The primary goal of root canal treatment (RCT) is to provide a hermetic seal to the root canal space with a GP cone and root canal sealer.[3] Several techniques highlighted the effectiveness of thermoplasticized GP under compaction.[8,9] Thus, viscoelasticity is an essential property for GP during compaction to allow a plastic deformation under continuous load leading to material flow.[10] A significant factor that affects the mechanical properties of GP is the prolonged storage of the material which may result in material brittleness and consequently affect their clinical utility.[11] GP becomes more brittle with prolonged exposure to air and light due to oxidation.[12] Thus, brittle GP cones are difficult to insert during root canal filling, and they have been always discarded. Moreover, the physical and mechanical properties of GP are affected by temperature.[13,14] In a study conducted by Goodman et al.,[13] an increase of 4°C in temperature was sufficient to produce softening of GP. Furthermore, temperature increase can enhance strength and resiliency, especially when the temperature exceeds 30°C.[10] Meanwhile, overheating up to 130°C causes permanent degradation and physical changes.[15] There is a lack of evidence in available dental research or another field of material research that verifies or elucidates if expired GP is brittle and cannot restore their properties. The aim of the present experiment is to test if expired GP loses its mechanical properties, to evaluate the effect of thermal treatments on expired GP, and to determine the changes that occur mechanically. The null hypothesis of this study is that there is a significant reduction in the tensile strength of expired GP compared to nonexpired one. In addition, GP that undergo thermal treatment restores its tensile strength compared to nontreated GP.

MATERIALS AND METHODS

This experimental laboratory study was approved by the Institutional ethical committee (04-12-19). Sample size calculations for this study revealed that at least 12 cones in each group should be utilized to detect a between-group 10% difference in the mechanical properties, with a power of 0.80 and a significance level equal to 0.05.[16] A total of 240 ProTaper GP Points X3/0.07 (Dentsply Sirona, Pennsylvania, USA) were selected. The cones were divided into five groups (n = 60). Group 1: nonexpired sealed GP, Group 2: nonexpired GP that was exposed to air and light for 30 days, Group 3: two years expired sealed GP, Group 4: 2 years expired GP that was exposed to air and light for 30 days, and Group 5: control group. The expired cones were chosen from King Abdulaziz University School of Dentistry and from private practices. Those GP cone packets, on which the dates of expiry or the date of “best before” was written, were included for study. In the expired groups, GP should be expired for about 2 years. Each group was divided into five subgroups according to the heat treatments and cooling temperatures. For first group (A), 12 cones did not receive any heat and were used as control; while for the second group (B),12 cones were heated individually for 1 min in a water bath kept at 90°C then quenched in tap water at room temperature (25°C) for 5 min. In the third group (C), 12 cones were heated individually for 1 min in a water bath kept at 90°C then kept for freezing at 0°C for 5 min, while in fourth group (D), 12 cones were heated individually for 1 min in a water bath kept at 70°C, then quenched in tap water at room temperature (25°C) for 5 min. In the last group (E), 12 cones were heated individually for 1 min in a water bath kept at 70°C then frozen at 0°C for 5 min. All specimens were exposed to tensile strength test using the universal testing machine (Instron MicroTester Precision Instruments, MA, USA). All cones were cut from the base to achieve a standardized 14 mm length. All cones were inserted from both ends to the holders of the universal testing machine at 2 mm from each side. The machine was operated according to the reference machine.[17] A fixed grip of 5 kN static load was equipped. The initial grip was set at 50 mm, while the crosshead speed was fixed at 4.8 mm/min. The tensile load was determined in all cones using the Series IX Automated Materials Testing System software (Instron Corporation, MA, USA). The data were analyzed using a one-way analysis of variance followed by multiple comparisons using Tukey's post hoc test. The significance level was set at 0.05. SPSS (Chicago, IL, USA version 22.0 for Windows) was used for statistical analysis of the data.

RESULTS

The results of this study expressed as means and standard deviations of the tensile strength of all groups are shown in [Table 1]. No difference in the tensile strength between expired and nonexpired GP was found. In addition, exposure of the GP to the air and light for 30 days did not show any changes in the tensile strength as compared to the sealed one. The mean of the tensile strength of nonexpired sealed GP was 1.52 ± 1.32 Mpa. This reading was similar to the other groups: nonexpired nonsealed (1.76 ± 1.32 Mpa at P = 1.00), expired sealed (1.42 ± 0.829 Mpa at P = 0.858), and expired nonsealed (1.06 ± 0.421 Mpa at P = 1.00). Heating the GP with a water bath at different temperatures produced softening of GP. The results exhibited a significant rise in the tensile strength in all groups heated for 1 min in a water bath at 70°C, especially with the nonexpired sealed GP, the mean of the tensile strength was 8.19 ± 2.72 Mpa and 5.94 ± 1.14 Mpa cooled at 25°C and 0°C, respectively, P ≤ 0.001. Yet, the tensile strength showed a significant reduction in all groups heated for 1 min in a water bath at 90°C, especially with the expired nonsealed cones, the mean of the tensile strength was 0.688 ± 0.917 Mpa, 0.262 ± 0.173 Mpa cooled at 25°C and 0°C, respectively, P ≤ 0.001. Cooling the GP at low temperature (0°C) produced a more brittle sample compared to those cooled at room temperature (25°C). However, it gets back to its phase once it gets out of the freezer, and no significant difference in the tensile strength between different cooling temperatures was observed.

Table 1.

The mean values (Mpa±standard deviation) of tensile strengths of gutta-percha at different conditions

Tested group	Mean (Mpa)±SD
	Nonexpired (sealed)	Nonexpired (nonsealed)	Expired (sealed)	Expired (nonsealed)
Control	1.52±1.32	1.76±1.32	1.42±0.829	1.06±0.421
Water bath 70°C/cooling at 25°C	8.19±2.72	5.67±1.42	5.25±0.786	4.612±0.337
Water bath 70°C/cooling at 0°C	5.94±1.14	4.20±1.47	4.831±564	3.960±0.404
Water bath 90°C/cooling at 25°C	0.78±0.456	0.735±0.307	0.913±0.497	0.688±0.917
Water bath 90°C/cooling at 0°C	0.40±1.06	0.65±1.01	0.78±0.280	0.262±0.173

SD: Standard deviation

DISCUSSION

This study aimed to assess if GP cones lose their tensile strength after being expired for 2 years and if exposure to air and light for 30 days may affect the mechanical properties of GP. It also evaluated the effect of different temperatures of heat baths in rejuvenating the GP. GP root canal filling is a composite material composed of pure GP (trans-1,4-polyisoprene), zinc oxide, barium sulfate, wax, or resin as a plasticizer.[18] The mechanical properties of GP are related to the amount of pure GP and zinc oxide.[19] Tensile strength was correlated to the amount of pure GP.[20] An increase in the tensile strength will enhance the clinical desirable characteristic of the cone and reflects the ability to retrieve a snugly fit inserted cone.[21] GP is a weak elastomer. It acts as a rubber polymer at room temperature, being above its glass transition temperature (Tg) that is about −60°C. All polymers have a special feature known as the glass transition temperature (Tg) that shows whether the polymer is brittle and hard or flexible and soft at a certain temperature. In this study, size X3/0.07 was selected to ensure a large size cone with good precision and accuracy during mechanical testing. Moreover, the tensile test was selected as the only test to determine if the GP cones are brittle and easily breakable. Other tests such as compressive strength, modulus of elasticity, or elongation were not considered as the GP cones are highly delicate materials. Based on the current result, 2 years of expiry assigned by the manufacturer was minimal to cause any change in the tensile strength. In addition, heat treatments to the cones may enhance the tensile strength. Yet, a further increase in the temperature may cause an adverse effect. Most clinicians discard the GP cones once they reach their date of expiry. This study shows that the 2 years expiration is short enough to cause any significant reduction in the tensile strength. A previous study reported that aged GP cone loses its mechanical properties;[12] the question that arises here is “how old is too old to cause a clinical change in the properties?” Further, this study advocated that exposing the GP cone to light and air for 1 month did not change the tensile strength. However, a study by Friedman[10] showed that prolonged exposure to light and air may negatively affect the mechanical properties. Factors that are dependent on the environmental storage such as prolonged exposure to air and light may cause a chemical degradation to the GP cone by oxidation hydrolysis and oxidative chain scission leading to changes in the crystallinity.[22] Degraded GP has poor plasticity due to polymer weight loss, allowing to decrease the apical sealing ability as reported by Gurgel-Filho et al.,[23] and could affect the RCT durability. Our study showed no changes between exposed and nonexposed groups. This could be attributed to the short duration of air and light exposure which is more relevant to the clinical situation. A study by Oliet and Sorin et al. showed that 250 days of air exposure causes brittleness.[24] In regards to heat treatment, a slight increase in the temperature of GP cones over its melting point enhances the tensile strength; yet, further study is required in this field to determine if this increase is permanent or for a short period of time. However, the increase in the temperature up to 90°C may adversely affect the properties. This is in accordance with the previous study.[15] Water absorption has a plasticizing effect on the dental GP, hence lowering its tensile strength. Although this phenomenon is not clear, it could be proposed to the chemical interaction and the insertion of water molecules and decrease the values of tensile strength. Lowering of the tensile strength is significant if water absorption occurs for 20 days or more.[25] However, this does not apply to our study because GP cones were soaked in water for only a minute to produce softening in the GP without causing any chemical interaction.

CONCLUSION

Within the limitations of the present study, all the investigated groups showed tensile strength equal to the nonexpired sealed group. Not only the expiration date of GP assigned by the manufacturers could influence the tensile strength but other important factors that are beyond the control, such as the amount of pure GP and storage condition of GP. Future study is required to test the effect of expired GP for a longer duration of more than 2 years and to test different mechanical properties. Appropriate storage of GP is a key factor to extend its lifespan. A clinician may consider using expired GP as long as there are no noticeable changes in their structure and properties. A simple chairside technique using hot water may be helpful in restoring the mechanical properties of expired GP.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.